The invention relates in general to measurements intended to determine at least one characteristic of oil well effluents made up of multiphase fluids, typically comprising three phases: two liquid phasesxe2x80x94crude oil and waterxe2x80x94and one gas phase based on hydrocarbons. The characteristics in question are specifically the proportions of the component phases, including the water content of the liquid phase, and the flow rate valuesxe2x80x94total flow rate and the flow rates of the various phases.
The ability of the oil industry to optimize production of a reservoir relies on the possibility of evaluating the well effluent at regular intervals, in terms of quantity (flow rate) and of composition (the proportions of the various phases). This makes it possible to determine what corrective action may need to be taken. However, measuring the flow rate of oil well effluent is a problem that is complex because of the way effluents are usually made up of three phases, and because of the changes in flow conditions to which they are subject (pressure, temperature, shape of pipes). These factors give rise to a wide variety of flow regimes being observed, including some regimes of highly non-uniform and unstable character, with the proportions of the phases in the fluid mixture being capable of varying very considerably both in the flow direction (i.e. over time) and across the flow direction, in the form of phase stratification across the flow section. One extreme, but very common, case is slug flow i.e. that of a high gas content with the flow being made up of an alternation of portions that are essentially gas, known as xe2x80x9cpocketsxe2x80x9d, and portions that are constituted essentially by liquid, known as xe2x80x9cplugsxe2x80x9d.
In the oil industry, the traditional practice is to separate the effluent into its component phases and to perform measurements on the phases separated in this way. However that technique requires separators to be installed on site, where separators are bulky and expensive items of equipment, and while wells are being tested, it also requires additional pipes to be put into place.
Numerous proposals have been put forward for developing techniques that would make it possible to avoid using such separators. A description of these developments is to be found in SPE publication 28515 (SPE Annual Technical Conference, New Orleans, Sep. 25-28, 1994) by J. Williars, xe2x80x9cStatus of multiphase flow measurement researchxe2x80x9d.
Most of the propositions suggest firstly a total flow rate sensor and secondly sensors for measuring the proportions of the phases in the mixture.
Amongst those proposals, U.S. Pat. No. 4,788,852 describes apparatus comprising a Venturi and a device for measuring gamma ray attenuation at three different energy levels, the device being situated at the constriction of the Venturi.
British patent application 2 128 756 explains that it is necessary to determine two of the following three magnitudes: total mass flow rate, total volume flow rate, and mean density of the fluid. In order to compensate for non-uniformities in the flow, which give rise in particular to differences of speed between the gas and the liquid phase, thereby making any flow rate measurement difficult, and also making density measurements inaccurate, it proposes homogenizing the fluid upstream from the sensors by means of an appropriate device. In addition to improving the quality of mean density measurement, homogenization has the effect of equalizing the speeds of the gas and liquid phases, and thus of enabling the gas flow rate to be measured.
An application of that principle is described in document WO 90/13859 which provides firstly a Venturi and a gamma ray density meter placed at the constriction of the Venturi, and secondly a mixer upstream from the Venturi for the purpose of homogenizing the multiphase fluid entering the Venturi. Nevertheless, the cost and the size of such apparatus may limit its commercial applications.
The invention seeks to characterize three-phase oil effluents by means that are simple and cheap, and that are applicable to a wide variety of flow regimes, and in particular to slug flow.
In one aspect, the invention provides a flow rate measurement method adapted to oil effluents made up of multiphase fluid mixtures comprising water, oil, and gas, the method comprising the following steps: the effluent is passed through a Venturi in which the effluent is subjected to a pressure drop; a mean value of the pressure drop is determined over a period t1 corresponding to a frequency f1 that is low relative to the frequency at which gas and liquid alternate in a slug flow regime; a mean value is determined for the density of the fluid mixture at the constriction of the Venturi over said period t1; and a total mass flow rate value  less than Q greater than  is deduced for the period t1 under consideration from the mean values of pressure drop and of density.
Appropriately, the density of the fluid mixture is measured by gamma ray attenuation at a first energy level at a frequency f2 that is high relative to said frequency of gas/liquid alternation in a slug flow regime, and the mean of the measurements obtained in this way over each period t1 corresponding to the frequency f1 is formed to obtain said mean density value.
Appropriately, the frequency f1 is 0.1 Hz or less than 0.1 Hz, e.g. 0.01 Hz. Appropriately, the frequency f2 is greater than 20 Hz, and preferably greater than 40 Hz, e.g. being equal to 45 Hz.